where presumably on the other side, there's a similar dropoff where it becomes plains land again.
I can confirm that. I live close to Montréal and if I travel to the north west, the plain turns into hills. That part is called the Laurentides and it's an old mountain range. I think it belongs to the Canadian Shield but the land is much more hilly than what lies further. The difference in elevation is strong enough to give ears pressure discomfort.
The Appalachian mountains is another old mountain range. It used to be very high but eroded over millions of years.
The area used to be very active (tectonic). But we are not at the edge of the plate anymore.
One thing that stand out in Eastern Canada and US is that the area was form by a successions on plate collisions, adding a bit more land every time.